1. Field of the Invention
The present invention relates to an image projection apparatus, and more specifically to a color image projection apparatus provided with a digital micromirror device as a display device and, for example, a laser light source as a light source for illuminating its image display surface.
2. Description of Related Art
As display devices for use in image projection apparatuses, there are known digital micromirror devices. A digital micromirror device has an image display surface composed of a plurality of minute mirrors; it controls the inclination of the individual mirror surfaces on the image display surface, thereby modulates the intensity of illumination light, and thereby forms an image. In a digital micromirror device, the turning on and off of each pixel is achieved, for example, by the rotation of a mirror surface through ±12° about a rotation axis at 45° to each side of the image display surface.
In recent years, digital micromirror devices with increasingly high definition have been developed to meet the needs for high-definition image projection apparatuses. As digital micromirror devices are given increasingly high definition, they come to have increasingly small pixel pitches, and this makes the influence of diffraction accordingly less negligible. Specifically, the smaller the pixel pitch is, the more a digital micromirror device acts as a diffraction grating, and, since incident light rays having different wavelengths have different diffraction angles, this results in uneven color in an image projected on a screen by using an image projection apparatus. The influence is particularly great in cases where a laser light source is used.
As one technology for reducing the influence of diffraction in digital micromirror devices, Patent Literature 1 listed below proposes a laser irradiation apparatus for laser machining. In this laser irradiation apparatus, to prevent lowered light transfer efficiency ascribable to diffraction, the image display surface of a digital micromirror device is inclined at a predetermined angle to the optical axis of an objective lens. Here, combining a diffraction angle with a geometric reflection angle makes it possible to reduce the influence of diffraction.
Patent Literature 1: JP-A-2010-44272
Unfortunately, however, it is difficult to apply the technology proposed in Patent Literature 1 to image projection apparatus. This is because, in an image projection apparatus, where proper imaging is expected over the entire screen, the quality of projected images will be degraded if the digital micromirror device is inclined at an angle large enough to reduce the influence of diffraction. For this reason, no image projection apparatus have thus far been proposed in which consideration is given to the influence of diffraction at a digital micromirror device.